From Akira TAGOH in downstream Red Hat/Fedora bug 2178389

Qt5 tries to gether font information through FcFontList() and register it into Qt's font database. and requesting a best font with FcFontMatch() against it. In fact, not requesting weight and width doesn't matter because it is actually linked to the font face and font index.

Let me explain a bit about variable fonts support in fontconfig. fontconfig has a special font pattern for variable fonts which has variable=true. this means "This is a variable font" and also contains some information which properties has "axis". applications can request some variations against it.

There are the miscalculation here. this pattern has index 0 and no values for properties which has "axis", because it will be figured out when the real axis is decided. thus, we don't have style for them.  However, Qt are going to fill in blanks with default values.  Thus, this pattern becomes Regular. Furthermore, Noto CJK fonts assigns Thin style to index 0 after updates. then, they dealt with Thin as Regular. that's it.